Tugging at the Future

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WRTSIL TECHNICAL JOURNAL 01.2010

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Tugging at the futureAU T HO R S: Ko en Vo nk, M a n a g e r S o l u t i o n E n g i n e e r i n g , S h i p P o w e r , W rt s i l i n C h i n a a n d

Le i f Bec ker , N a v a l A r c h i t e c t , P r o j e c t M a n a g e r , W r t s i l S h i p D e s i g n i n N o r w a y

Te most common purpose o tugs is toassist other vessels during harbour entryand manoeuvering. o perorm this taskeectively they need to be compact, highlymanoeuverable and powerul. Te deningcharacteristic that determines the tugsincome is its maximum bollard pull; inother words, the pulling orce that a tug canexert on a stationary vessel. All tug systemsare thereore designed to cost-eectivelyachieve the highest possible classication-certied rating.

For W UG, we have ocused on a

typical harbour tug having a bollard pullo 60 tons, and an LNG terminal escorttug with a bollard pull o 80 tons. Teseare reerred to as the W UG 60 andW UG 80 respectively.

Design briefTe standard o todays modern tugsresults rom experience gained over manyyears by designers, owners, yards,equipment suppliers and classicationsocieties. Recognizing this key aspect,the W UG project solicited valuablecomments rom several tug operators andequipment suppliers throughout the process.Knowledge gained rom joint industryprojects, like SAFEUG, has also beenimplemented. Vessel perormance, ease oproduction, ease o maintenance as well as

saety were selected as being the key actorsin developing a vessel compliant withtomorrows environmental and economicrequirements.

Te W UG 60 design targets typical

harbour duties. Compact size, highmanoeuvrability, and re ghting capabilityare required or this tug with a bollard pullo 60 tons and 12.5 knots trial speed.Te total cost should be within reach o alloperators, including those in areas with

low margins such as developing markets.Accommodation is specied or seven men.

Te design brie or the W UG 80requires sae operation in exposed areaslike oshore terminals. Escorting at highspeed, push-pull operations and coastaltowing are also typical tasks. Tis calls ora highly manoeuvrable vessel with goodsea-keeping characteristics. Te vessel alsoneeds re ghting capability, the ability tooperate 200 nautical miles rom the coastand a relocation range o 4000 nauticalmiles. Bollard pull capacity is 80 tons and

the trial speed is 14 knots. Te coastalaspect requires accommodation or eightmen.

The W TUG project is a Wrtsil initiative aimed at developinga new concept for tugs. It brings together the companys Ship Designand Ship Power expertise and aims to design a tug to meet tomorrowsenvironmental and economic requirements.


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Fig. 1a The W TUG 60 Fig. 1b The W TUG 80, hybrid version

Main parameters:

W TUG60 W TUG80

Length over all LOA 29.5 m 35.40 m

Max breadth 11.8 m 14.00 m

Depth hull (excl skeg) (approx.) 5.6 m 6.65 m

Draft below skeg (approx.) 5.4 m 6.20 m


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The W TUG standardTe W UG 60 has one basic outt level

suitable or its intended harbour operations.Tere is a towing winch on the oredeckand a towing hook on the at deck. Fireghting monitors are located atop thewheelhouse. Te specication allows ormedium-speed Wrtsil 9L20 enginesor high-speed alternatives.

Te W UG 80 has a basic and anelaborate deck outt option. Te basicversion includes a oredeck winch, intendedor towing and escorting duties, withtowing pins on the bow. A second towingwinch is located on the at deck together

with a deck crane and capstan. In theextended outt a stern roller, towing pins,towing hook and tugger winch complementthe basic outt. Fire ghting equipmentis located atop the wheelhouse.

Te engine conguration consists o twoalternatives. Te rst is the conventionaltwin medium-speed engine congurationwith Wrtsil 8L26 engines. Te alternativeis an advanced hybrid version, with twinWrtsil 9L20 main engines and a singleWrtsil 6L20 generating set connectedto the thrusters by means o an electrical

power take in. More details on this hybridversion are presented later in this article.

In the ollowing part o this article wewill ocus on two key elements, optimizationo both the hull and the hybrid machinery.

Designing with CFDSeveral types o CFD (computational uiddynamics) are used in parallel during thedesign process. Panel methods are used toanalyze ship motions, whilst hull resistanceis investigated by means o the RANSmethod. By utilizing CFD, model test

results can be predicted with close to 90%accuracy. Te optimization o hull shapesand appendages thus starts well beoremodel building and tank testing. Tis savestime and the combination o methodsprovides greater understanding o the owphenomena around the hull and appendages.

During the optimization process manydierent congurations are analyzed indierent operating conditions. CFDcalculations oten allow unique, colourulviews on the vessel, as can be oundthroughout this article.

Fig. 2 CFD investigation of the flow along the hull during free sailing.


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Te optimization o the hull appendages,like the skeg, is now ongoing as best

illustrated in Figure 3. As mentioned, thisgoes hand in hand with comprehensivemodel testing, including towing, selpropulsion (orward and at), escorting,bollard pull and manoeuvring tests. Pitchand roll characteristics will be conrmed, asrequired by the design brie, or operatingthe W UG 80 in exposed sea areas. Temodel tests are essentially the conrmationo the results obtained by CFD.

Machinery optimizationTodays tug

ypically, tugs operate at an average engineloading o 20% or around 2000 hours peryear (Figure 4 and Figure 5). Te maximumbollard pull (100% power) is seldom useddespite it being a design driver. Althoughthe exact operating proles dier betweenharbours, this trend is conrmed by keyplayers in the market. Such low actualengine loading is ar removed rom thedesign condition o most engines. Ideallytugs should thus be optimized or bothlow power and the ull engine rating.

Average loading is only part o the story.

When assisting another vessel, poweravailability is just as important since ullpower may be required at a moment'snotice. At the other extreme, engine loadduring transit is very stable and predictable.

Reliability, power availability andinvestment considerations have resultedin straightorward diesel-mechanicalinstallations. Tese continue to servethe industry well, but can be improvedupon rom environmental and efciencyperspectives.

Tomorrows tug Hybrid

Te simultaneous optimization o both lowand high power output is possible with ahybrid machinery conguration. Such aconguration has been applied to manyvessel types, such as oshore supply vesselsrom which the proven technology has beenborrowed. Te nal conguration appliedor the W UG is shown in Figure 6 withboth mechanical and electrical power trains.Te electrical input is simply mounted onthe reverse side o the thruster as per

existing dual input installations. Te targetor this hybrid is to be able to transit on asingle engine in diesel-electric mode, andthen supplement this with the mechanicalengines when in assist mode. Te sizing

Fig. 3 CFD in escorting mode. In this calculation the tug is pulled directly towardsthe viewer and generates a steering force on the large vessel being assisted.

Fig. 4 Distribution of engine load over time.

Fig. 5 The operating profile.

Engineload

Operating profile

100%

80%

60%

40%

20%

0

0 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

time [%]

operating profile average engine load

Operating profile

25%

20%

15%

10%

5%

0

Relativeoperationtime[%]

Increasing power

Loitering/standby

Assist10%

MCR

Transit15%

10knots

Assist25%

MCR

Transit30%

11.5knots

Assist60%

MCR

Assist100%

MCR


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Fig. 6 The hybrid machinery configuration. The Wrtsil 9L20 engines are mechanically coupled to the CS300 thrusterunits by means of a shaft. The components in the blue box are the heart of the hybrid. The electrical power take in (PTI) andfrequency drive unit take electrical power from the Wrtsil 6L20 generating set and the small harbor generating set.The PTIs can be used to run the thrusters independently in a pure electrical mode or boost the mechanically supplied power.The batteries in the red box can be added to create a completely silent electrical operating mode.

Fig. 7 Energy consumption per operating condition.

o the engines thus starts rom the bollardpull requirement, resistance curve, and theelectrical load balance. Te aim is tooptimize the resistance curve and equipmentef ciency or the same transit operatingspeed. A key operator concern is to be ableto get out o the way o an assisted vesselin case o a black-out on the tug. Tisintrinsic ailsae o mechanically drivenvessels is taken into account in the hybrid.In act a second, purely electrical ailsae is

introduced in case o a mechanical ailure.An indication o where savings can be

achieved is shown in the overall energyconsumption or electrical and propulsiveneeds (Figure 7).

ransit between the berth and the areao assist represents almost a third o allenergy consumption. Te main enginesoperate at less than 30% load in thiscondition. Applying more powerdoes not result in a signicant speedincrease, but merely generates excessivewaves. ogether, these acts indicate

the potential or improvement.Loitering, with the engines mostly

idling, is typically an inef cient operatingcondition. At least part o the 8% energyconsumption should be salvageable.

Electric cross link

Batteries 1000 kW(option)

PTI 1000 kW

CS300-3000 HR

CS300-3000 HR

PTI 1000 kW

Frequency drive

WRTSIL 9L20

WRTSIL 6L20

HS Harbour

WRTSIL 9L20

1800 kWm at 1000 rpm

1055 kWe at 900 rpm

250 kW at 1800 rpm

1800 kWm at 1000 rpm

Harbour

Loitering/standby

Assist 10%

Assist 25%

Assist 60%

Assist 80 TBP

Transit 15% 10 knots

Transit 30% 11.5. knots

Transit 30% 11.5. knots23%

Harbour 15%

Loitering/standby8%

Assist 10%10%

Assist 25%28%

Assist 60%6%

Assist 80 TBP3%

Transit 15% 10 knots7%


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Perhaps surprisingly, the harbour powerrequirements contribute 15% to total

energy consumption. Tis is not becauseo the high power demand, but is dueto the large amount o time spent in theharbour. ypically one o the harboursets is always running at high rpm and atrelatively low loads during much o its lie.

Te route to the propeller is dierentin the case o mechanical and electrictransmission. Te efciency comparisonhas thus been perormed based on uelconsumption per kW o power deliveredto the thruster, thus taking the ull chaino efciencies into account. Tis provides

an interesting insight into the relativeperormance o dierent power sources,including batteries, high-speed engines andmedium-speed engines. At the low loads atwhich tugs operate, a 20% dierence inuel oil consumption can be ound betweenthe best and worst congurations. Te samemethod can be used or optimizing anemissions prole.

Besides selecting engines with low specicuel oil consumption in the relevantoperating areas, a lot can be gained or lostin the combination o propeller rpm and

propeller pitch. Te combination o rpmand pitch over the operating range isgenerally reerred to as the combinator.Low thrust requirements can be achievedwith high rpm/low pitch or low rpm/highpitch. Te latter is typically more efcientor the same thrust when using a nozzle,but is limited by the minimum rpmrequirements o diesel engines. Electricmotors allow operation at extremely lowrotation rates. Electrically driving the mainpropulsion at low loads is thus used tobenet hydrodynamic efciency and to

reduce mechanical losses in the gears andbearings. Such hydrodynamic efciencyimprovements more than oset the lossesintroduced within the electrical system inseveral operating conditions. Te capabilityo stopping the propellers altogetherimproves efciency even urther.

Te hybrid solution presented inFigure 6 is the result o comparingmany dierent installations. It achievesan efciency improvement o just over10% compared to a conventional twinmedium-speed engine tug by optimizing

the integrated system. Tis efciencyimprovement is evenly spread out overall operating conditions, bar the harbourand loitering/standby conditions. Tiseven spread is an indication o the low

sensitivity o the hybrid congurationto the exact operating prole. In theloitering/standby operating condition, thesavings can be up to 50% depending onthe interval at which propulsion poweris required. I harbour power is sourcedrom the harbour generating set, then

no signicant savings can be achieved.Reducing the harbour generating set size tothe point where it could not satisy totalonboard demand was not considered anoption. Alternatively, the power requiredin harbour can be sourced rom shore.Depending on the type o shore based powerplants in use, this could provide a urtherefciency and emissions improvement.

Even i the maximum bollard pull ratingis rarely used, it needs to be availableinstantaneously during ship assist operations.o achieve this response, all equipment

required to reach the vessels bollard pullrating is running during the assist mode.Tis, unortunately, means some o theequipment is still operating at low loadingto meet harbour saety requirements. Te

presented savings in the assist mode are,thereore, conservative.

ControlsTe strong demand or, and limitedavailability o, skilled crew has resulted in areduction in crew size. Many operators are

running their vessels with only three crewmembers, while the industry is discussingthe implications o two man crews.

With crew size in mind, the hybridmachinery must be easy to use. Te tugmaster simply requires his attention inorder to saely perorm the vessel assistancetask at hand. Te applied user interacehas, thereore, been designed to onlyrequire input that comes naturally to thetug master. He can recognize whether hisvessel is in transit or about to assist anothervessel, or example. Selection o these

operations-related conditions by the tugmaster allows the vessel controls to optimizethe machinery conguration or optimalefciency and emissions. It should be notedthat the changeover between operating


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conditions is not required during peakactivity levels, but solely at moments whenthe tug master can devote a small amounto his attention. Such a simple user interaceis essential to materializing the calculatedimprovements.

Behind the user interace, the controls

also provide the intelligence. For theselected operating condition, certainengines are brought online in keeping withthe power requirements normally expectedin that operating condition. Te thrustercontrol levers unction as normal,triggering the normal power and steeringcommands. Handling o the vessel is thusnot aected. In case the power demandsignal is higher than the power available,the tug master is prompted to switch toa higher level o power availability. Similarprompts occur in case o a sub-optimal

system conguration or an extendedperiod o time. Note that such a promptwill not interere with vessel assist operationswhen the tug master needs his ull attention.

Fuel oil consumption and emissionsIntroducing the hybrid or the presentedoperating prole reduces uel consumptionby just over 10% compared to theconventional twin medium-speed engineset up. CO2 and SOXemissions are directlyrelated to uel consumption and are reduced

by the same amount. NOXemissions arereduced by just over 12% as a result o theimproved engine operating points madepossible by the hybrid concept. In case ashore power connection is used or harbourpower requirements, an additional 15%o the reerence vessels energy consumptioncan be transerred to land-based powerplants where exhaust ater-treatment iseasier to implement.

FutureBatteries, dual-uel engines, and uel cells

allow urther emission and efciencyimprovements. Even more advanced hybridtugs can be expected in the near uture.

CONCLUSIONAt Wrtsil, we believe that emissionregulations require the re-design o manyexisting vessel types. Tis is especially trueor vessels operating in the proximity olarge population centers, as is the casewith tugs in ports. Te steadily decreasing

cost and high reliability o availabletechnology indicate that the marine sectoris ready or more advanced solutions.Te economic viability o these complexsolutions typically results rom an integratedapproach, rom vessel design to equipmentselection and on-site support orthe shipyard and owner.

Wrtsils approach is to be the singlepoint o contact during the design,building, and even operational phases,thus providing the W UG customerassurance that this advanced vessel solution

is supported at every stage o its liecycle.

Tugging at the Future

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